1. Field of the Invention
The present invention relates to an antireflection film as a coating over a surface of an optical element and, more particularly, to an antireflection film effective against ultraviolet light of the wavelengths not more than 250 nm.
2. Related Background Art
The antireflection films for the ultraviolet light, using fluoride films, are described in Japanese Patent Laid-Open Application Nos. 61-77001, 7-244205, and 7-244217.
The Japanese Patent Laid-Open Application Nos. 7-244205 and 7-244217 disclose NdF3, LaF3, etc. as high-index fluoride films. These films, however, have a problem that they are inferior in environment resistance (characteristic stability under high temperature and high humidity environments) to oxide films such as Al2O3.
It is thus difficult to apply such films to a projection optical system using a lot of lenses like a refracting projection optical system used in exposure apparatus (so called steppers) for projecting and printing a reticle pattern onto a semiconductor wafer in a semiconductor fabrication step. In the case of the projection optical system using such many lenses, even if change is small in the antireflection characteristics of each surface due to time-dependent change, the total change of the overall projection optical system would result in a large characteristic shift.
An object of the present invention is thus to provide an antireflection film of a wide bandwidth excellent in the environment resistance performance (stability of optical characteristics with a lapse of time) in the ultraviolet wavelength region, an optical element with the antireflection film, and a method for producing the antireflection film.
The inventors found that use of Al2O3 film as a high-index film permitted production of the densest film among high-index film materials available in the ultraviolet wavelength region and considerably improved the environment resistance performance (the temporal stability of optical characteristics) of the antireflection film.
We also clarified that films of AlF3 including the same metal Al were preferably used as low-index films to be alternately stacked with Al2O3 and that loss was small at the interfaces. It was also clarified that MgF2 could replace AlF3.
Good results were obtained when the film-forming methods were vacuum evaporation; and dense films were able to be formed by sputtering, whereby the environment resistance performance was improved. In addition, a uniform film can also be produced on a curved surface of a lens or the like by Chemical Vapor Deposition (CVD).
In view of expansion of the antireflection bandwidth and thickness errors due to manufacturing errors, an antireflection film of the present invention for accomplishing the above object is an antireflection film comprising:
an alternately multilayered film having a designed center wavelength xcex0 in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index nh and low-index layers of a refractive index nl in order from the side of a substrate to the side of air, said alternately multilayered film having a four-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions;
1.35xe2x89xa6nlxe2x89xa61.45
1.60xe2x89xa6nhxe2x89xa61.85
nlxe2x89xa6nxe2x89xa6nh; and
wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d1, d2, d3, and d4 in order from the first layer to the fourth layer when counted from the substrate side, the first layer and the third layer are the high-index layers, the second layer and the fourth layer are the low-index layers, and the optical thicknesses satisfy the following conditions:
0.38 xcex0xe2x89xa6d1xe2x89xa60.43 xcex0
0.38 xcex0xe2x89xa6d2xe2x89xa60.43 xcex0
0.20 xcex0xe2x89xa6d3xe2x89xa60.25 xcex0
0.20 xcex0xe2x89xa6d4xe2x89xa60.25 xcex0.
Another antireflection film of the present invention is an antireflection film comprising:
an alternately multilayered film having a designed center wavelength As in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index nh and low-index layers of a refractive index nl in order from the side of a substrate to the side of air, said alternately multilayered film having a five-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions;
1.35xe2x89xa6nlxe2x89xa61.45
1.60xe2x89xa6nhxe2x89xa61.85
nlxe2x89xa6nxe2x89xa6nh; and
wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d1, d2, d3, d4, and d5 in order from the first layer to the fifth layer when counted from the substrate side, the first layer, the third layer, and the fifth layer are the low-index layers, the second layer and the fourth layer are the high-index layers, and the optical thicknesses satisfy the following conditions:
0.42 xcex0xe2x89xa6d1xe2x89xa60.47 xcex0
0.37 xcex0xe2x89xa6d2xe2x89xa60.42 xcex0
0.37 xcex0xe2x89xa6d3xe2x89xa60.42 xcex0
0.20 xcex0xe2x89xa6d4xe2x89xa60.25 xcex0
0.20 xcex0xe2x89xa6d5xe2x89xa60.25 xcex0
Still another antireflection film of the present invention is an antireflection film comprising:
an alternately multilayered film having a designed center wavelength xcex0 in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index nh and low-index layers of a refractive index nl in order from the side of a substrate to the side of air, said alternately multilayered film having a six-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions;
1.35xe2x89xa6nlxe2x89xa61.45
1.60xe2x89xa6nhxe2x89xa61.85
nlxe2x89xa6nxe2x89xa6nh; and
wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d1, d2, d3, d4, d5, and d6 in order from the first layer to the sixth layer when counted from the substrate side, the first layer, the third layer, and the fifth layer are the high-index layers, the second layer, the fourth layer, and the sixth layer are the low-index layers, and the optical thicknesses satisfy the following conditions:
0.40 xcex0xe2x89xa6d1xe2x89xa60.45 xcex0
0.37 xcex0xe2x89xa6d2xe2x89xa60.42 xcex0
0.45 xcex0xe2x89xa6d3xe2x89xa60.50 xcex0
0.04 xcex0xe2x89xa6d4xe2x89xa60.09 xcex0
0.28 xcex0xe2x89xa6d5xe2x89xa60.33 xcex0
0.20 xcex0xe2x89xa6d6xe2x89xa60.25 xcex0
The high-index layers are Al2O3 as described above and the low-index layers are preferably AlF3 or may be MgF2.
The present invention has the following effects.
(1) The present invention was able to provide the antireflection films of the wide bandwidth in the ultraviolet region by the above-stated film structures, using Al2O3 for the high-index layers and AlF3 or MgF2 for the low-index layers.
(2) The present invention was able to provide the antireflection films in the ultraviolet wavelength region, excellent in the environment resistance, by using Al2O3 for the high-index layers.
(3) The present invention was able to provide the antireflection films for ultraviolet light using the Al2O3 layers, which were superior in the environment resistance to those constructed only of fluoride layers.
(4) The present invention was able to provide the antireflection films for ultraviolet light of the wide bandwidth in the four-layered, five-layered, and six-layered structures by optimization of the film configuration, and the production method thereof.